This invention relates to a porous grinding tool and method for grinding a roll to a mirror finish. More particularly, it relates to a porous grinding tool and method suitable for grinding a gravure printing copper-plated roll.
Copper-plated rolls for use in gravure printing are typically in the form of a solid or hollow cylindrical iron core which is copper plated. The roll is ground at its surface with a porous grinding tool to a mirror finish such as to enable satisfactory gravure plate-making and printing. One typical roll grinding process is described with reference to FIG. 1. A copper-plated roll 1 for gravure printing is rotated about its axis at a predetermined rotational speed. A porous grinding tool 2 is placed in contact with the surface of the rotating roll 1 while the grinding tool 2 is rotated abut its axis and moved, preferably back and forth, along the axial direction of the roll. In this way, the surface of the roll 1 is ground to a mirror finish with the grinding tool 2. For surface grinding of the roll 1, a wet grinding procedure is often employed. Specifically, the load applied to the grinding tool 2 is adjusted so that the grinding tool 2 may contact the surface of the roll 1 over an appropriate area and under an appropriate pressure, and the position of the grinding tool 2 relative to the roll 1 is properly adjusted. The grinding tool 2 and the roll 1 are rotated about their respective axes at appropriate rotational speeds and the grinding tool 2 is moved back and forth along the axial direction of the roll 1 while water is being sprayed to the grinding zone from a nozzle (not shown) disposed alongside the roll.
In the prior art, the grinding tool 2 is configured as a disk or short cylinder as shown in FIG. 2, typically having a diameter of about 200 mm and a thickness of 50 to 100 mm. The grinding tool 2 is provided at its center with a through-hole 3. A hollow rotating arbor 4 (FIG. 1) is connected to the back surface of the disk-shaped grinding tool 2 so as to surround the through-hole 3. The abrasive surface of the grinding tool 2 is perpendicular to the rotating arbor 4. The through-hole 3 in the grinding tool 2 and the bore of the rotating arbor 4 are used to draw off and discharge grinding debris from the grinding zone.
In the above-described grinding method, the rotational axis of the roll is perpendicular to the rotational axis of the grinding tool. The prior art grinding method using a porous grinding disk ensures that the gravure printing copper-plated roll on its surface is ground to a mirror finish at a high dimensional precision. However, feed marks can be left on the roll surface due to the feed rate of the grinding disk. The use of a grinding disk has the effect of leaving on the surface of the roll, upon completion of grinding, grinding marks in the form of a plurality of circumferentially extending, generally parallel and closely spaced streaks as shown in FIG. 3. Gravure printing using a roll bearing such streaky grinding marks results in impressions bearing streaky defects, failing to meet the requirements of the current art for high precision, high quality printed matter.
An object of the invention is to provide a porous grinding tool for grinding a roll to a mirror finish at a high dimensional precision without leaving streaky feed marks of a large inclination angle on the surface of the roll. Another object of the invention is to provide a method for grinding a roll using the grinding tool.
We have found that a porous grinding tool for use in grinding of a roll is improved by changing the planar shape of at least its working surface from the conventional circular shape to a polygonal shape having from four (4) to twenty (20) sides. The grinding tool with a polygonal working surface is successful in grinding a roll at a high dimensional precision without leaving feed marks due to the tool feed as streaks parallel to the circumferential direction of the roll, but as streaks of a small feed mark pitch inclination angle. When the grinding tool is applied to a roll for gravure printing the roll can be ground to a sufficient finish to produce printed matter without streaky defects.
In a first aspect, the invention provides a porous, roll-grinding tool having an abrasive surface adapted to contact and grind the surface of a roll. The abrasive surface is configured to a planar polygonal shape having from four (4) to twenty (20) sides.
A second aspect of the invention provides a method for grinding the surface of a roll having an axis, comprising the steps of rotating the roll about the roll axis, and placing a porous grinding tool in contact with the surface of the rotating roll while rotating the grinding tool about an axis thereof and moving the grinding tool along the axial direction of the roll, thereby grinding the roll surface with the grinding tool. The grinding tool has an abrasive surface which is configured to a planar polygonal shape having from 4 to 20 sides.